1. Field of the Invention
This invention relates to a switching circuit comprising first and second switches operable to produce an electrical signal at the output of the switching circuits at certain voltages. Moreover, this invention also relates to a method of operating such a switching circuit.
The switching circuit may comprise a bridge circuit operated to produce a voltage across a load connected to an output of the bridge circuit in accordance with a required voltage. For example, a presently contemplated application of the present invention is in driving an electromagnet in response to a demanded force produced by the electromagnet that must be met to a very high tolerance. The force produced by the electromagnet can be controlled in response to either current demands or voltage demands, because control of either current or voltage affects the force produced by the electromagnet. In addition, the current controller can be operated in response to force demands, although this must be converted into a current or voltage demand. Even where the current controller is operated in voltage demand mode with demanded voltages being set across the electromagnet, this will of course influence the current flowing through the electromagnet and so the term ‘current controller’ is used to cover operation in response to voltage or current or field demands.
Where the current controller is operated in response to voltage demands, high-frequency voltage pulses may be applied to the electromagnet because the large inductance associated with an electromagnet leads to a slow response time in the current so that it smoothly follows drifts in the average voltage applied across the electromagnet.
A known switching circuit is shown in FIG. 1. As can be seen, the switching circuit comprises a half-bridge circuit with an electromagnet connected across its output. Control of the voltage across the electromagnet is achieved by switching a pair of transistors positioned on opposed arms of the bridge circuit (the other opposed arms containing diodes to complete the half-bridge circuit) to alter the polarity of the voltage across the electromagnet between +VS and −VS. A current or voltage demand will be received and switching signals are generated to match this demand. The switching signals are supplied at the points marked A and B to control the transistors such that they are switched between maximum and minimum conducting states (their linear region is not used due to poor power efficiency). The diodes in the half-bridge circuit ensure that current flows in one direction only through the electromagnet, in this case from right to left in FIG. 1. The current controller is operated such that the transistors are switched concurrently: when both transistors are on (i.e. conducting), a voltage of +VS is applied across the electromagnet and when the transistors are off (i.e. non-conducting), a voltage of −VS is applied across the electromagnet. The duty cycles at +VS and −VS determine the average current delivered to the electromagnet in that period, remembering that the inductance of the electromagnet ensures that the current smoothly follows the voltage rather than sharply jumping with each voltage pulse. Hence, by switching the transistors at appropriate times, the desired current can be delivered to the electromagnet. A reservoir capacitor is included to hold current drawn from the electromagnet that cannot be passed back to the DC supply.
The pulsed voltage signal that produces these duty cycles is implemented using pulsed switching signals supplied to the transistors. The switching signals are modulated according to a pulse width modulation scheme according to an analogue-implemented scheme, such that the width of the pulses within a period are varied so that the pulse at +VS is varied relative to the remaining time at −VS to produce the desired current. Alternatively, a pulse density modulation scheme may be used, as is well known in the art.
2. Discussion of Prior Art
A problem with the above described switching circuit is that there are a number of offsets in voltages across various parts of the circuit that will cause the voltage produced across the electromagnet to differ from that required. These voltage offsets may be caused by fluctuations in the DC supply about the nominal value of +VS (both due to mains ripple and the voltage across the reservoir capacitor varying as it charges and discharges, in particular) and by voltage drops across the diodes and transistors caused by their intrinsic resistance. In addition, slow response times in the components causes slow variation in the voltage supplied to the electromagnet that falls behind the required variation. If these effects are not corrected for, the average voltage delivered to the electromagnet during the period will not match the required voltage.